1. Field of the Invention
This invention relates to rectifiers and more particularly, to those rectifiers whose inputs are connected to a three phase source.
2. Description of the Prior Art
Rectifiers include internal power supply circuits, also known as auxiliary supplies, to provide the operating voltages for the electronics used in the rectifier. When the rectifier is connected to a three phase source of power, the internal power supply circuit is connected to two of the phases. If either or both of those phases are lost, then the operation of the rectifier is inhibited as the auxiliary supply cannot provide the internal operating voltages.
A problem does, however, occur when the phase to which the internal supply circuit is not connected is lost. That phase will be referred to hereinafter as "the unconnected to phase". When the loss of that phase occurs, the operation of the rectifier should also be inhibited. As the internal supply circuit has in the past been connected only to the two non-lost phases, there isn't any loss of internal operating voltages as a result of the loss of the unconnected to phase.
One solution used in the past to inhibit the operation of the rectifier when the unconnected to phase is lost has been to turn the rectifier off. While that does provide the desirable result of turning off the rectifier when the unconnected to phase is lost, it does require that the rectifier be turned on when that phase is restored.
The rectifier typically has its three phase input connected through a full wave bridge to a capacitor bank. When all three phases are operational, a relatively low amplitude three phase ripple current having a frequency of 360 Hz flows through the capacitor bank. When any one of the three phases is lost, the amplitude of the ripple current increases substantially and has a frequency of 120 Hz. The ripple current amplitude may exceed the rating of the capacitors in the bank.
Other solutions to the loss of the unconnected to phase have involved limiting the current flow through the rectifier to keep it within the rating of the bank capacitor(s) or increasing the rating of the bank capacitor(s). The former increases the complexity of the rectifier, while the latter is quite expensive to implement.
It is desirable to have an internal power supply circuit which not only inhibits the operation of the rectifier when either of the two phases to which the internal supply circuit is connected are lost but also when the unconnected to phase is lost. It is also desirable that such a supply circuit be relatively easy to implement and not very costly as compared to the conventional supply circuit. The loss protection circuit of the present invention meets these requirements.